1. Field of the Invention
This invention is a method to reduce levels of mercury in process water and wastewater to very low concentrations.
2. Description of the Prior Art
Many industrial processes require water with very low concentrations of metals. In addition, federal guidelines mandate that mercury-containing wastewater to be discharged from hospitals, clinical and industrial laboratories, groundwater redemption facilities, or manufacturing plants into the environment must contain very low levels of mercury contaminants. For wastewater, required discharge concentrations are frequently in the range of 1-10 micrograms Hg/L, and are lower in some places. It is also believed that this process will be of use in removing mercury from polluted groundwaters.
Mercury has a strong tendency to form both ionic and covalent complexes with a wide variety of materials that may be present in water, including particulate or colloidal material, organic molecules, and inorganic ions. Mercury is particularly susceptible to the formation of chelate complexes with polydentate organic ligands. Mercuric ion can also undergo spontaneous reduction to the metallic form. Mercury in these complexed or metallic forms can be highly resistant to removal by conventional treatment means.
Conventional treatment methods for the removal of mercury include amalgam formation with silver or gold, adsorption on activated carbon, adsorption on a cation exchange material, precipitation with sulfide ion, or adsorption on resins having immobilized chelating groups.
As described by Capuano et al. in U.S. Pat. No. 4,230,486, formation of a mercury amalgam is carried out by contacting a mercury-containing solution with particles of metallic silver. The mercury dissolves in the silver to form an amalgam and a purified liquid. The amalgam can then be heated to vaporize the mercury, allowing both the silver and the mercury to be recovered. However, organic mercury compounds do not readily react with silver, as noted by Yan in U.S. Pat. No. 5,322,628. Yan further reports that gold will readily remove organic mercury species; however, the amount of gold required to successfully reduce mercury levels in industrial effluents renders such a process commercially unfeasible.
Adsorption of mercury on activated carbon is often used to remove mercury from liquids. However, the mechanism of carbon adsorption of mercury is not well understood, and the technology is often unable to meet low discharge limits. The adsorbent occasionally releases large quantities of adsorbed mercury at unpredictable times. This can cause dangerous levels of mercury to be released into the environment accidentally.
Treatment of wastewater containing mercury with a cation exchange material is known to reduce mercury concentration. However, ion exchange of mercury-bearing wastewater is unable to reduce the mercury concentration to the required levels without the use of extremely large adsorbent columns, due to the relatively low equilibrium binding constant between the mercury ions and the ion-exchange adsorbent.
Precipitation of mercury with insoluble hydroxides or sulfides with alkali, lime, or sulfide ion is also known. However, such techniques can be difficult to use on a small scale, and are unable to meet discharge limits without complex and expensive microfiltration systems. Also, industrial effluents often contain organic chemicals which form soluble complexes with mercury ions. These complexes are often highly resistant to precipitation.
Resins or inorganic matrices having immobilized chelating groups have been developed for removal of mercury from wastewater. Many of these materials contain immobilized hydroxyl groups. These resins or matrices can reduce levels of mercury in wastewater to the desired discharge levels with ionic mercury solutions. For example, Holbein et al., in U.S. Pat. No. 4,752,398, when wastewater is allowed to flow through a silica gel carrier having immobilized cysteine groups bound thereto at flow rates of 30-95 ml/hr, the mercury concentration in the wastewater may be reduced from 14 ppm to less than 0.03 ppm. Similarly, exposure of mercury-containing solutions to vinyl polymers having immobilized thiosemicarbazido groups has been found to reduce mercury concentrations from 200 ppm to under 0.01 ppm (Motani et al., U.S. Pat. No. 3,847,841).
However, some difficulties have been experienced with these chelating materials. First, sulfhydryl-based resins are extremely susceptible to oxidation. Also, these resins are unable to reduce mercury levels to the desired levels with wastewater feed streams that contain high levels of complexed mercury. Finally, even when these resins are effective, the kinetics of mercury adsorption onto the resin are very slow.
Attempts have been made to overcome some of these problems. Cremers et al., in U.S. Pat. No. 4,167,481, disclose that mercury levels in a mercury-containing solution containing an anionic organic ligand such as EDTA or citrate can be reduced in a two-step process. First, the solution is treated with a polyamine which tightly complexes the mercury ions, displacing any anionic ligands present to form more stable cationic complexes. The solution is then treated with an ion exchange material such as an aluminosilicate which adsorbs polyamine complexes of mercury. However, this has the disadvantage that it depends on the binding constants between polyamines and mercury; if a chelating group which binds as strongly or more strongly to mercury than polyamines is present, the system will fail to sufficiently reduce mercury levels.
Another method of removing metals from an aqueous solution containing organic metal-complexing materials by treating the solutions with an oxidant has been proposed by Alvino et al. (U.S. Pat. No. 5,564,105). A solution containing organic complexes of ferrous cations may be treated with an oxidizing agent which oxidizes the organic complexing agent to water and carbon dioxide, and causes the contaminating ferrous material to separate from the solution as a precipitate. The oxidation step takes from 20 to 50 minutes, and is carried out at a temperature of between 140xc2x0 F. and 212xc2x0 F. Almost as an afterthought, Alvino et al. suggest that it may be possible to treat mercury-containing solutions using this process as well. However, the oxidation takes place under elevated temperatures, and it requires a substantial energy input to maintain these temperatures. A process that allows one to oxidize a solution containing organic complexes of metal cations to produce free metal cations at ambient temperatures without increasing reaction times would be greatly desired.
It is an object of the invention to develop a method of dramatically reducing mercury ion concentrations in process waters and wastewaters containing mercury. This method should be effective at removing elemental mercury, ionic mercury, covalent mercury compounds, or chelate mercury complexes.
More particularly, the invention relates to a method of removing mercury from wastewater, comprising the steps of treating wastewater containing mercury-containing contaminants, such as ionic mercury, elemental mercury, covalent mercury compounds, and mercury complexes of organic and/or inorganic ligands, with an oxidizing agent to produce pretreated wastewater, and passing the pretreated wastewater through an adsorbent material which adsorbs or binds free mercury ions. The oxidizing agent oxidizes elemental mercury to ionic mercury, and releases complexed mercury ions into a non-complexed ionic form. These ions may then be more readily adsorbed by the adsorbent material, which is preferably a resin or inorganic matrix which has mercury-selective chelating groups attached thereto. These and other methods will apparent in light of the following figures and detailed description.